髓鞘轴突(神经纤维)通过动作电位有效地在整个大脑中传递信号。对轴突方向敏感的多种方法,从显微镜到磁共振成像,目的是重建大脑的结构连接体。当数十亿的神经纤维在每个点上以各种可能的几何形状穿过大脑时,解决纤维交叉是必要的,以生成准确的结构连接图。然而,有特异性地这样做是一项具有挑战性的任务,因为来自定向纤维的信号会受到与有髓鞘轴突无关的脑(微)结构的影响。由于髓鞘的周期性,X射线散射可以特异性探测髓鞘轴突,在散射图案中产生不同的峰。这里,我们表明,小角度X射线散射(SAXS)可用于检测髓鞘,轴突特异性纤维交叉。我们首先展示了使用人类call体条带创建人工双交叉和三交叉纤维几何形状的能力,然后我们在鼠标中应用该方法,猪,Vervet猴子,和人类的大脑。我们将结果与偏振光成像(3D-PLI)进行比较,示踪剂实验,以及扩散磁共振成像的输出,有时无法检测到交叉。鉴于其特殊性,3维采样能力和高分辨率,SAXS可以作为验证使用扩散MRI以及基于显微镜的方法得出的纤维取向的基本事实。重要声明:为了研究我们大脑中的神经纤维是如何相互连接的,科学家需要可视化他们的轨迹,它们经常互相交叉。这里,我们展示了独特的能力小角度X射线散射(SAXS)来研究这些纤维交叉不使用标签,利用SAXS对髓磷脂的特异性-包裹在神经纤维周围的绝缘鞘。我们使用SAXS来检测双重和三重交叉纤维,并在小鼠中揭示复杂的交叉,猪,Vervet猴子,和人类的大脑。这种非破坏性方法可以发现复杂的纤维轨迹,并验证其他不太具体的成像方法(例如,MRI或显微镜检查),对动物和人脑中的神经元连接的准确映射。
Myelinated axons (nerve fibers) efficiently transmit signals throughout the brain via action potentials. Multiple methods that are sensitive to axon orientations, from microscopy to magnetic resonance imaging, aim to reconstruct the brain\'s structural connectome. As billions of nerve fibers traverse the brain with various possible geometries at each point, resolving fiber crossings is necessary to generate accurate structural connectivity maps. However, doing so with specificity is a challenging task because signals originating from oriented fibers can be influenced by brain (micro)structures unrelated to myelinated axons. X-ray scattering can specifically probe myelinated axons due to the periodicity of the myelin sheath, which yields distinct peaks in the scattering pattern. Here, we show that small-angle X-ray scattering (SAXS) can be used to detect myelinated, axon-specific fiber crossings. We first demonstrate the capability using strips of human corpus callosum to create artificial double- and triple-crossing fiber geometries, and we then apply the method in mouse, pig, vervet monkey, and human brains. We compare results to polarized light imaging (3D-PLI), tracer experiments, and to outputs from diffusion MRI that sometimes fails to detect crossings. Given its specificity, capability of 3-dimensional sampling and high resolution, SAXS could serve as a ground truth for validating fiber orientations derived using diffusion MRI as well as microscopy-based methods. STATEMENT OF SIGNIFICANCE: To study how the nerve fibers in our brain are interconnected, scientists need to visualize their trajectories, which often cross one another. Here, we show the unique capacity of small-angle X-ray scattering (SAXS) to study these fiber crossings without use of labeling, taking advantage of SAXS\'s specificity to myelin - the insulating sheath that is wrapped around nerve fibers. We use SAXS to detect double and triple crossing fibers and unveil intricate crossings in mouse, pig, vervet monkey, and human brains. This non-destructive method can uncover complex fiber trajectories and validate other less specific imaging methods (e.g., MRI or microscopy), towards accurate mapping of neuronal connectivity in the animal and human brain.